Exhaust treating system for lean-burn CNG engine

ABSTRACT

A three-way catalyst system for efficiently converting the exhaust gas from a CNG-fueled engine when operated under lean conditions at a redox ratio of 0.02-0.9, the system comprising a first stage catalyst comprising a transition metal-containing zeolite; means for injecting a hydrocarbon into said exhaust gas prior to entry of said exhaust gas into said first stage catalyst, said hydrocarbon having a greater affinity than CH 4  in its ability to react with NO; and a second stage catalyst for treating the effluent from said first stage catalyst and comprising a high surface area alumina impregnated with discontinuous La 2  O 3  and palladium. 
     The invention also comprehends a method of treating exhaust gases from a lean-burn CNG-fueled engine, operating at a redox ratio of 0.02-0.9; exposing such exhaust gases to a first stage catalyst consisting of copper-ZSM5 zeolite having at least 3% by weight ion-exchange copper; injecting a fast-acting hydrocarbon into said exhaust gas prior to entry of the exhaust gas into said first stage catalyst, the hydrocarbon having a greater affinity than CH 4  in its ability to react with NO; and exposing the effluent from the first stage catalyst to a second stage catalyst comprising palladium supported on alumina containing discontinuous La 2  O 3 .

This is a divisional application of U.S. Ser. No. 07/789,707 filed Nov.8, 1991.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to the technology of treating exhaust from acompressed natural gas (CNG) fueled engine to remove its noxiouscontent, and more particularly to the treatment of exhaust from a CNGengine controlled to operate under lean-burn combustion conditions.

In copending U.S. Ser. No. 07/789,707, filed Nov. 8, 1991, authored bysome of the authors of this invention and commonly assigned to theassignee herein, a catalyst is disclosed which enhances the three-wayconversion capability of a modified Pd/Al₂ O₃ catalyst in treating theexhaust gas of a compressed natural gas-fueled engine, provided theengine is limited to being operated slightly rich of stoichiometry,i.e., redox ratio (R) of 1-1.2 (R being the ratio of reducing componentsto oxidizing components in the exhaust gas). Untreated exhaust from aCNG-fueled engine, operated under rich conditions, contains a highcontent of CO (about 2000-2250 ppm), a high content of NO_(x) (at leastabout 450 ppm), and a methane content at least about 300 ppm. Althoughthe enhancement achieved by this disclosure over the prior art issignificant, fuel-rich operation affects the fuel economy of theCNG-fueled engine and therefore can be undesirable. At stoichiometry orbelow stoichiometry (i.e., lean region), the conversion capability ofsuch a catalyst drops dramatically.

If the exhaust gas is pretreated by use of a copper-exchanged zeolite,prior to entering the three-way/CNG catalyst described above, the enginecan be operated at stoichiometry to achieve conversion efficiency inexcess of 80% for all of CO, NO, and CH₄ (see copending U.S. Ser. No.07/789,558, now U.S. Pat. No. 5,197,053 authored by some of the authorsof this invention, and commonly assigned to the assignee herein).However, if this combination is used to treat the exhaust from afuel-lean operated CNG engine, the conversion capability drops againdramatically. Moreover, expensive electronic controls are required toregulate the engine operation at stoichiometry.

2. Discussion of the Prior Art

Copper-exchanged zeolites have been used to cleanse lean-burn typeexhaust, but only from exhaust gases simulating the exhaust from aconventional gasoline-fueled engine. Such gasoline engine exhaustcontains very high contents of fast-burning hydrocarbons, arepresentative of which is propylene (at about 1000 ppm), high contentsof slow-burning hydrocarbons, a representative of which is propane (atabout 500 ppm), very high contents of NO (about 1000 ppm), and very highcontent of CO (at about 15,000 ppm), with an absence of methane. Acopper-exchanged zeolite catalyst would not be effective, by itself, intreating the total exhaust from a CNG-fueled engine operating under leanconditions, since such an exhaust would contain considerably loweramounts of NO and CO but significant amounts of methane. The conversionefficiency would be well below 80% (see Li et al, "StoichiometricCatalytic Decomposition of Nitric Oxide Over Cu-ZSM-5 Catalyst", Journalof Physical Chemistry, Vol. 94, p. 6145, 1990; Iwamoto et al, "Influenceof SO₂ On Catalytic Removal of NO Over Copper Ion-Exchange ZSM-5Zeolite", Applied Catalysis, Vol. 69, L 15-L 19, 1991; and Hamada et al,"Highly Selective Reduction of Nitrogen Oxides With Hydrocarbons OverH-Form Zeolite Catalysts In Oxygen-Rich Atmospheres", Applied Catalysis,Vol. 64, L 1-L 4, 1990).

What is needed is a catalyst system that economically and durablyconverts CO, NO_(x), and CH₄ present in the exhaust of a lean-burnCNG-fueled engine.

SUMMARY OF THE INVENTION

The invention artificially injects a fast-burning hydrocarbon into theexhaust gas of a CNG-fueled engine (such hydrocarbons may naturallyoccur in the exhaust gas of gasoline-powered engines), such injectionmodifying the content of the exhaust gas of a lean-burn CNG engine priorto entering into a zeolite-type first stage catalyst. The effluent fromsuch first stage catalyst contains an exhaust gas that is criticallychanged in character prior to entering the second or downstream catalyststage, the NO and CO having been considerably reduced and oxidizedrespectively allowing the second stage catalyst to focus primarily uponCH₄ conversion.

More particularly, the invention is a three-way catalyst system forefficiently converting the exhaust gas from a CNG-fueled engine whenoperated at lean conditions a redox ratio, R=0.02-0.9, the systemcomprising: (a) a first stage catalyst comprising a transitionmetal-containing zeolite; (b) means for injecting a hydrocarbon intosaid exhaust gas prior to entry of said exhaust gas into said firststage catalyst, said hydrocarbon having a greater affinity than CH₄ forreacting with NO; and (c) a second stage catalyst for treating theeffluent from said first stage catalyst and comprising a high surfacearea alumina impregnated with lanthana and palladium.

The invention also comprehends a method of treating exhaust gases from aCNG-fueled engine, the method comprising: (a) operating the engine underlean-burn conditions with redox ratio of 0.02-0.9; (b) exposing suchexhaust gases to a first stage catalyst consisting of copper-ZSM5zeolite having at least 3% by weight ion-exchange copper; (c) injectinga fast-burning hydrocarbon into said exhaust gas prior to entry of theexhaust gas into said first stage catalyst, the hydrocarbon having agreater affinity than CH₄ for combining with NO; and (d) exposing theeffluent from said first stage catalyst to a second stage catalystcomprising a gamma alumina support impregnated with palladium and othercatalytic activity and durability enhancing oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the preferred form of elements ofthe catalytic system of this invention;

FIG. 2 is a graphical illustration of percent conversion efficiency as afunction of R value for a catalyst system that has the first stage andsecond stage catalyst of this invention but is operated without theinjection of a fast-burning hydrocarbon such as propylene prior to entryof the exhaust gas into the first stage;

FIG. 3 is a graphical illustration of percent conversion efficiency ofNO as a function of the injected propylene concentration;

FIG. 4 is a graphical illustration of percent conversion efficiency forthe system of this invention as a function of the redox ratio at theinlet of the first stage catalyst;

FIG. 5 is a graphical illustration of percent conversion efficiency forthe system of this invention as a function of R value when the propyleneinjection is held constant and oxygen varied; and

FIG. 6 is a graphical illustration of percent conversion efficiency forCH₄ and NO as a function of R when the CNG exhaust (no C₃ H₆ added) isonly exposed to the first stage catalyst.

DETAILED DESCRIPTION AND BEST MODE

In the catalytic system of this invention as shown in FIG. 1, the firststage comprises a zeolite-based catalyst, preferably copper-ZSM5,followed by a second stage comprised of palladium supported on La₂ O₃/Al₂ O₃ composite oxide where La₂ O₃ a is preset as a discontinuousphase.

Zeolite Catalyst

The catalyst contains a transition metal-containing zeolite; the zeoliteis desirably a high silica zeolite having a SiO₂ /Al₂ O₃ molar ratiothat exceeds 10, preferably up to about 60 (see U.S. Pat. No. 4,297,328,which is expressly incorporated herein by reference, for teaching use ofother class of zeolites.

The transition metal, such as copper, is provided into the zeolite byion-exchange. The transition metal may be selected from the groupconsisting of Cu, Co, Ni, Cr, Fe, Mn, Ag, Zn, Ca, and compatiblemixtures thereof. Generally, a sodium, hydrogen, or ammonium zeolite iscontacted by an aqueous solution of another cation, in this case anaqueous solution of soluble copper compound such as copper acetate,wherein replacement of the sodium, hydrogen, or ammonium ion by copperion takes place. It is advantageous to provide as much transition metalion in the zeolite as possible since the amount of transition metalpresent in the zeolite is directly related to the catalytic activity ofthe first stage. Preferably, this is at least 3% by weight of zeolite,up to a maximum determined by the SiO₂ /Al₂ O₃ ratio. After replacingthe sodium, hydrogen, or ammonium ion with the metal ion, the zeolite isgenerally washed to remove excess surface transition metal compound. Itis not necessary to do so, however.

The first stage catalyst may also contain a transition metal-containingoxide, but such transition metal should be of the same type as that usedin the ion exchange for the zeolite. Preferably, this transition metalis copper and copper is particularly preferred because it is active atlower temperatures. Preferably, the oxide is zirconia and the metal itcontains is copper, although other oxides such as titania, silica, andvery minor proportions of lanthana aluminate may be employed. One methodof making a copper-containing zirconia comprises soaking a quantity ofzirconia, in the form of a fine powder, repeatedly, if desired, in asolution of copper compound. The copper impregnated ZrO₂ is subsequentlydried and calcined at temperatures between 300°-600° C., often at about450° C. The copper compound should be one that is soluble or that can bedispersed in a liquid, that is, those which are soluble in an aqueoussolution or which can be solublized therein, e.g., with the aid of anacid or base. Exemplary of such copper compounds are copper salts likecopper nitrate and copper sulfate; organo-copper compounds likecarboxylates of copper, copper acetate, and copper-cupric amines;organo-complexes of copper like diamine copper acetate; tetraaminecopper sulfate, and . copper acetylacetonate. Soluble compounds,exemplary of other transition metal compounds include cobalt acetate,nickel acetate, ferric chloride, chromic nitrate, and manganese acetate.

The saturated zirconia is then dried and calcined in air, the coppercompound decomposing to form copper oxide. Preferably, copper is presentin an amount between 1-20% by weight as CuO. Each of thecopper-containing oxide and the copper-containing zeolite may be groundto a fine powder and mixed together to form a slurry. The slurry is thendeposited on a substrate such as a metal or ceramic honeycomb. While itis preferable to make the catalyst this way, it may also be made bylayering one material over another.

CNG/Three-Way Catalyst

The second stage catalyst functions to cleanse the exhaust effluent fromthe first stage when operated under lean-burn engine exhaust conditions.The catalyst comprises a high surface area gamma alumina support whichis impregnated with 0.5-20% La₂ O₃ or its equivalent. Palladium in anamount 0.2-30% by weight of the second stage catalyst is impregnated onthe La₂ O₃ /Al₂ O₃ support. The operation of such a second stage will bedescribed with that optimum catalyst in place. The support is preferablyalumina of the gamma form rather than of the delta or alpha formsbecause the gamma form provides, among other factors, a greater surfacearea. With gamma alumina, the surface area will be significantly higherand be in the range of 50-400 m² /gm. The particle size of the gammaalumina should be preferably less than 200 angstroms and the monolithcarrier should have a cell size in the range of 100-600 cells per squareinch. Gamma alumina may also be modified with oxides of base, rareearth, and alkaline metal such as barium, cerium, titanium, and nickelto promote thermal stability, catalytic activity, durability, andwashcoat adhesion.

The lanthana impregnation is carried out to load the support withlanthana in the weight range of 0.5-20%. If lanthana is added in anamount less than such range, then the beneficial effect of increase inactivity due to lanthana addition is not observed. If lanthana exceedssuch range, then the support surface area decreases and no additionalbenefit is derived. It is important that the lanthana be applied in adiscontinuous mode to the support so that both the palladium andlanthana are simultaneously exposed to the exhaust gas. Elements thatare partial equivalents to the function of lanthana for purposes of thisinvention may include tungsten oxide and molybdenum oxide. Theconversion efficiency enhancement will be less with either of the latteroxides; therefore, it is desirable if only a portion of La₂ O₃ isreplaced by WO₃ or MoO₃.

Palladium is impregnated in a manner to provide the presence of largecrystalline particles, preferably in the particle size range of 20-1000angstroms. Hence, the Pd weight loading is in the range of 0.2-30%. Withpalladium weight loadings below 0.2%, there will be an insufficientcatalysis effect and therefore not promote the objects of thisinvention. If the palladium loading is in excess of 30%, the palladiumsurface area decreases and no additional benefit from palladium additionis derived.

Other elements that may be present in the second stage catalyst mayinclude elements that avoid retention of water for improving thelong-life stability of catalysts. This may include elements such astungsten oxide (incorporated by using ammonium meta tungstate during theimpregnation process) or chromium oxide, both of which tend to preventoxidation of palladium by reducing the mobility of water and therebykeeping it away from the palladium.

Performance

Samples of the catalyst system of this invention were prepared. Thefirst stage catalyst was formed by using a commercially available ZSM5zeolite catalyst and contacting it with an aqueous solution of coppernitrate (under controlled pH) to exchange 5% by weight of copper. Theresulting material was dried at 120° C. The 5% Cu/ZSM5 powder wassuspended in an aqueous slurry and deposited on a monolithic cordieritesubstrate. The resulting material was dried and calcined at 450° C. toform the 5% Cu/ZSM5 catalyst.

The second stage catalyst was prepared by using a washcoated monolithiccordierite substrate containing predominantly gamma alumina andrelatively small amounts of alpha alumina, nickel oxide, cerium oxide,lanthana, and titania; the substrate was dipped in an aqueous solutionof lanthanum nitrate to discontinuously deposit 10% lanthana by weightof the washcoat system. The substrate was dried at 120° C. and calcinedat 600° C. The substrate was then dipped in an aqueous solution ofpalladium chloride containing 4% by volume HNO₃ to deposit 1% palladiumby weight of the washcoat system. The precursor was dried at 120° C. andcalcined at 600° C. to form a three-way catalyst.

The catalyst system was first analyzed in a flow reactor underconditions used to simulate CNG vehicle exhaust without the injection ofany HC reductant: 300 ppm CH₄, 2250 ppm CO, 750 ppm H₂, and 425 ppm NOat 550° C. The O₂ concentration was varied and N₂ was used as thecarrier gas.

The results, as exhibited in FIG. 2, show that the NO conversionefficiency drops to 0% at R values lower than 0.8. Thus, the CNG-fueledengine would have to be operated around the stoichiometric point forefficient removal of the three main constituents without injection of anadditional reductant; this is undesirable from a fuel economystandpoint. When varying amounts of propylene reductant are injectedinto the feed gas fed into such catalyst system, the NO conversionefficiency reaches at least about 80% when 1600 ppm or more of C₃ H₆ ispresent, as shown in FIG. 3. The R value of the raw exhaust gas for FIG.3 test conditions was 0.052; when the propylene is injected, the R valueat the inlet to first stage catalyst will be slightly higher, but thisis taken into account in FIG. 4. In FIG. 4, close to 100% conversionefficiency is maintained for CO and CH₄ even when C₃ H₆ is injected as areductant through the lean combustion region (0.02-0.9 R). But, mostimportantly, conversion efficiency of NO is dramatically increased frombelow 3% to an excess of 80% in the lean-burn region. Also, all of theadded propylene (essentially 100%) is converted to CO₂ and water vapor.This is a surprising result because CH₄ conversion actually drops withincreasing R value in the lean region when exposed to the first stagecatalyst only (see FIG. 6); moreover, NO conversion efficiency remainslow.

The effect of varying oxygen concentration is shown in FIG. 5. Here theamount of propylene added was held constant at 2470 ppm and the oxygenconcentration was varied. The redox ratio refers to the condition at theinlet of the catalyst (after addition of propylene). The hydrocarbon andcarbon monoxide conversions slightly decrease as the amount of oxygenpresent decreases (increase in redox ratio). The nitric oxide conversionincreases with a decrease in the amount of oxygen present.

The addition of propylene increases the nitric oxide conversion in thelean region significantly. Also, propylene is fully converted to carbondioxide and water vapor. This observation may be explained as follows.The exhaust gas with added propylene flows over the zeolite catalystfirst. The reactions that are catalyzed ##STR1## C₃ H₆ also reacts withNO, N₂, CO₂, and H₂ O, and species obtained by partial oxidation of C₃H₆ are obtained as products.

Briefly, the zeolite catalyst allows a fraction of the added propyleneto react with nitric oxide; nitrogen, carbon dioxide, and water vaporare among the products obtained. In addition, the zeolite catalystoxidizes (i) propylene (with oxygen) and methane to carbon dioxide andwater vapor, and (ii) carbon monoxide to carbon dioxide. The CNGthree-way catalyst placed further from the engine manifold converts theunconverted hydrocarbons (including methane and propylene), oxygenatedcompounds, and carbon monoxide from the first stage to carbon dioxideand water vapor by reactions (2)-(4). To summarize, removal of nitricoxide is accomplished by its reaction with propylene over the zeolitecatalyst. The oxidation of methane, propylene, and carbon monoxideoccurs on both catalysts.

Any CNG exhaust system known to date does not provide high nitric oxideconversion in the fuel lean region. These exhaust systems limit thepotential of CNG engines by requiring the engines to be calibrated inthe fuel-rich region or require that the air/fuel ratio be tightlycontrolled. The additional of hydrocarbons, such as propylene, ethylene,or propane, results in high conversions for all three constituents,nitric oxide, methane, and carbon monoxide, under fuel-lean conditions.A lean-burn CNG engine can effectively utilize the high "octane" ratingof the CNG fuel and thereby offer superior engine performance in termsof fuel economy and power output considerations.

                  TABLE I                                                         ______________________________________                                        CNG                GASOLINE                                                   ______________________________________                                        CH.sub.4                                                                              300 ppm        C.sub.3 H.sub.8                                                                         500 ppm                                                             C.sub.3 H.sub.6                                                                       1,000 ppm                                      NO      425 ppm        NO      1,000 ppm                                      CO      2250 ppm       CO      15,000 ppm                                     H.sub.2 /CO                                                                           0.5-0.33       H.sub.2 /CO                                                                           0.33                                           SO.sub.2                                                                              up to 5 ppm    SO.sub.2                                                                                 20 ppm                                      ______________________________________                                    

We claim:
 1. A method of treating exhaust gas from a CNG-fueled internalcombustion engine, comprising the steps of:(a) operating said engineunder lean-burn conditions at a redox ratio of 0.02-0.9, the exhausthaving HC constituted essentially of methane; (b) exposing the exhaustgases from said engine to a first stage catalyst comprising a transitionmetal-exchanged zeolite; (c) injecting a fast-burning hydrocarbon intothe exhaust gas prior to entry of such exhaust gas into the first stagecatalyst, said hydrocarbon having a greater affinity than CH₄ in itsability to react with NO; and (d) exposing the effluent from the firststage catalyst to a second stage comprising palladium and discontinuousLa₂ O₃ supported on alumina.
 2. The method as in claim 1, in which saidexhaust gas exposed to each of said first and second stage is in thetemperature range of 400°-750° C. and the space velocity through each ofthese catalyst stages is in the range of 2-100 K hr⁻¹.
 3. The method asin claim 1, in which said method is effective in converting NO presentin the said exhaust gas by at least 80%, and in converting the CH₄ andCO present in the said exhaust gas by at least 97%.
 4. The method as inclaim 1, in which said first stage catalyst is prepared by mixingzeolite with Al₂ O₃ in a form selected from the group consisting ofpowder and sol.
 5. The method as in claim 1, in which said exhaust gasesgenerated in step (a) comprise NO no greater than about 425 ppm, CO nogreater than about 2250 ppm, and CH₄ no greater than about 300 ppm. 6.The method as in claim 1, in which said zeolite is Cu-ZSM5.
 7. Themethod as in claim 1, in which said transition metal for said zeolite isselected from the group consisting of Cu, Co, Ni, Cr, Fe, Mn, Ag, Zn,and Ca.
 8. The method as in claim 1, in which said fast-burninghydrocarbon is selected from the group of propylene, ethylene, andpropane.